Photoionization of the hydrogen atom near the ionization limit in strong electric fields

The Stark spectroscopy of the hydrogen atom has been investigated near the ionization threshold in static homogeneous external electric fields from ∼1000 to ∼8000 V/cm with use of a two-step excitation H(n=1)+ħ${\omega }_{\mathrm{vuv}\to }$H(n=2)+ħω ${}_{\mathrm{uv}\to }{}^{}{}_{}{}^{}$ ${\mathrm{H}}^{\mathrm{*}}$ with vuv and uv laser radiation, independently tunable and linearly polarized either parallel (π) or perpendicular (σ) with respect to the external field. Different from other, nonhydrogenic atoms, the hydrogen atom is a peculiar case insofar as the initial states of H(n=2), from which excitation takes place, are essentially of parabolic character at the field strengths employed in this study.